Operation of five individual diode lasers as a coherent ensemble by fiber coupling into an external cavity

Five diode lasers that each have one facet antireflection coated were fiber coupled into an external cavity and operated as a coherent ensemble by the use of a spatial filter placed at the Fourier plane of the cavity. The resulting output was shown to be in a single spectral line with a linewidth less than the instrumental resolution (7.5 MHz) of the Fabry–Perot spectrum analyzer used. The effect of rotating the polarization of the radiation inside one of the fibers is described.